1. Field of the Invention
The invention relates to an electroluminescent device of compound semiconductor and more particularly to a structure of an electroluminescent device of pn junction type comprising II-VI compound semiconductor of larger bandgap and being capable of emitting light in wavelength blue to ultraviolet.
2. Description of the Related Art
A conventional light emitting diode made of II-VI compound semiconductor and adapted to emit light in visible short wave range does, as shown in FIG. 26, generally comprise a substrate 101 made of GaAs(100), n type ZnSe:Ga layer 102 and p type ZnSe:O layer 103 each formed on the substrate in this order and is provided with a negative electrode Al 100 and a positive electrode Au 104 (as disclosed, for example, in K. Akimoto et al. Japan. J. Appl. Phys. 28(1989) L2001). The blue light emitting diode of the conventional structure emits blue light of peak wave 440 nm at low temperature of liquid nitrogen (77K, -196.degree. C.) but does provide almost no emission at room temperature (25.degree. C.).
The conventional electroluminescent device, in particular, the electroluminescent device made of II to VI compounds semiconductor which is expected (and able) to emit light with high luminous efficiency places the foregoing ZnSe layer, which emits blue light, directly on the GaAs substrate. Hence, crystals of the layers have difference (about 0.3%) in lattice constant, so that defect of very high density is introduced into the ZnSe epitaxial growth layer to cause the same layer to be largely deteriorated in crystallizability.
The defect created above substantially decreases luminous intensity 440 nm (77K) in bandgap of ZnSe when the layer is not applied with any impurity. Gallium (Ga) may be added as n type impurity as used in the conventional example to reduce density of the defect and increase the luminous intensity 440 nm. Since completeness of crystals depends in principle on lattice constant of the epitaxial junction structure of the crystals, defect due to heteroepitaxial growth still remains much in crystals except the case using a thin film structure comprising an ultra thin film.
Crystallizability of ZnSe:O formed by adding, for example, oxygen (O) onto the above heteroepitaxial growth layer of low quality is poor. The ZnSe:O epitaxial layer serving as p type layer for applying current cannot provide a sufficient amount of carriers (positive holes) in ZnSe:Ga emitting layer for emission; is not evaluated precisely by Hall Measurement Method; and is an impurity added film of quite low luminous efficiency wherein hole density is to be one thousandth of added oxygen atoms. This is caused not only by the fact that oxygen is not p type impurity in valency with respect to selenium but also that oxygen is poor in crystallizability.
Similarly, other conventional examples of the similar structures which formed on GaAs to provide p type epitaxial layer by using ZnSe:Li,N (T. Yasuda et al. Appl. Phys. Lett.52(1988)57) or ZnS:N (M. Migita et al, Abstracts. 4th I.C. on II-VI compounds, II-VI' 89, Fr-3-3) merely exhibit the properties of the same degree as the above example. Hence, such problems hindering the industry in development have been revealed that the devices when used as light emitting diode are deteriorated in a short time, do not emit light at room temperature and is poor in reproducibility for manufacture.
The problems of the conventional electroluminescent device of II-VI compound semiconductor are summarized as follows:
(1) Quality of crystals of a light emitting layer is low due to inconformity of lattice constant between crystals of substrate and epitaxially grown layers.
(2) Although a layer (generally of p type) for current application formed as epitaxial crystals is more sensitive in their properties to a structure (defects) than a light emitting layer is, this layer is poor in crystallizability.
(3) Hence, the devices are low in efficiency of current application and recombined emission, unstable in emitting properties and poor in reproducibility, and the whole properties do not at all reach a practical use level.
As seen from the above, it is decisive on practical use of the device in the industry that the essential technology to prepare electroluminescent devices including high luminance blue light emitting diode has not been well established.
An object of the present invention is to provide an electroluminescent device of II-VI compound semiconductor having a composition which can solve the problems revealed in the above conventional electroluminescent devices of II-VI compound semiconductor.